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I. 269 CT Withstand
When is withstand important?
Withstand is important when the phase or ground CT
has the capability of driving a large amount of current
into the interposing CTs in the relay. This typically oc-
curs on retrofit installations when the CTs are not sized
to the burden of the relay. (New electronic relays have
typically low burdens (2 mΩ for 269), while the older
electromechanical relays have typically high burdens (1
Ω).)
For high current ground faults, the system will be either
low resistance or solidly grounded. The limiting factor
that determines the amount of ground fault current that
can flow in these types of systems is the capacity of the
source. Withstand is not important for ground fault on
high resistance grounded systems. On these systems,
a resistor makes the connection from source to ground
at the source (generator, transformer). The resistor
value is chosen such that in the event of a ground fault,
the current that flows is limited to a low value, typically
5, 10, or 20 Amps.
Since the potential for very large faults exists (ground
faults on high resistance grounded systems excluded),
the fault must be cleared as quickly as possible. It is
therefore recommended that the time delay for short
circuit and high ground faults be set to instantaneous.
Then, the duration for which the 269 CTs subjected to
high withstand will be less than 250ms (269 reaction
time is less than 50ms + breaker clearing time).
NOTE: Care must he taken to ensure that the inter-
rupting device is capable of interrupting the poten-
tial fault. If not, some other method of interrupting
the fault should be used, and the feature in ques-
tion should be disabled (e.g. a fused contactor re-
lies on fuses to interrupt large faults).
The 269 CTs were subjected to high currents for 250ms
bursts. The CTs were capable of handling 500A for
short bursts. 5OOA relates to a 100 times the CT pri-
mary rating. If the time duration required is less than
250ms, the withstand level will increase.
II. CT Size and Saturation
How do I know how much current my CTs can out-
put?
CT characteristics may be acquired by one of two
methods.
The rating (as per ANSI/IEEE C57.13.1) for relaying
class CTs may be given in a format such as these:
2.5C100, 10T200, T1OO, 10C50, or C200. The num-
ber preceding the letter represents the maximum ratio
correction; no number in this position implies that the
CT accuracy remains within a 10% ratio correction
from 0 to 20 times rating. The letter is an indication of
the CT type. A 'C' (formerly L) represents a CT with a
low leakage flux in the core where there is no apprecia-
ble effect on the ratio when used within the limits dic-
tated by the class and rating. The 'C' stands for
calculated; the actual ratio correction should be differ-
ent from the calculated ratio correction by no more than
1%. A 'C' type CT is typically a bushing, window, or
bar type CT with uniformly distributed windings. A 'T'
(formerly H) represents a CT with a high leakage flux in
the core where there is significant effect on CT per-
formance. The 'T' stands for test; since the ratio cor-
rection is unpredictable, it is to be determined by test.
A 'T' type CT is typically primary wound with unevenly
distributed windings. The subsequent number specifies
the secondary terminal voltage that may be delivered
by the full winding at 20 times rated secondary current
without exceeding the ratio correction specified by the
first number of the rating. (Example: a 10C100 can
develop 100V at 2Ox5A, therefore an appropriate ex-
ternal burden would be 1Ω or less to allow 20 times
rated secondary current with less than 10% ratio cor-
rection.) Note that the voltage rating is at the secondary
terminals of the CT and the internal voltage drop
across the secondary resistance must be accounted for
in the design of the CT. There are seven voltage rat-
ings: 10, 20, 50, 100, 200, 400, and 800.
comes close to a higher rating, but does not meet or
exceed it, then the CT must be rated to the lower value.
The curve in Figure H.1 represents a typical excitation
curve for a CT. The Y-axis represent secondary exciting
voltage; the X-axis represents the secondary exciting
current. When the CT secondary exciting voltage level
is picked off the graph, the corresponding secondary
exciting current is the amount of current required to
excite the core of the CT. With respect to the ideal CT
that conforms perfectly to its ratio, the exciting current
could be considered loss.
For a Protection Class CT with a 5A secondary and
maximum 10% ratio error correction, it is probable that
the design point for 20 times rated secondary will be at
or slightly lower than the 10 amp secondary exciting
current point (10% of 20x5A) . To design such that the
20 times rated secondary current is in the linear region
would be more expensive.
In order to determine how much current CTs can out-
put, the secondary resistance of the CTs is required.
This resistance will be part of the equation as far as
limiting the current flow. This is determined by the
maximum voltage that may be developed by the CT
secondary divided by the entire secondary resistance,
CT secondary resistance included.
The easiest method of evaluating a CT is by the Exci-
tation Curves Method, as illustrated by the curves
shown in Figure H.2. The Y-axis represents secondary
exciting voltage; the X-axis represents the secondary
APPENDIX H
If a CT
H-1
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